1. Field of the Invention
This invention relates to quantum non-demolition optical tapping of particular, but not exclusive, application to communications systems in which a transmitted data optical signal is to be accessed by many receiving stations.
2. Related Art
A commonly employed method of transmitting data from a transmitter station to many receiver stations is to convert the data into a modulated optical signal and transmit it down an optical fibre. Each receiver station is associated with an optical tap which extracts some of the optical energy in the transmitted signal which it can subsequently convert to an electrical signal for processing by means of an optical detector. The optical tap may be an optical fibre coupler for example.
A disadvantage of such optical taps is that the transmitted optical signal is reduced in intensity by each successive optical tap. This can be avoided by using quantum non-demolition (QND) optical taps, that is, optical taps in which the tapping process does not significantly affect the intensity of the transmitted signal.
An experimental arrangement for the QND determination of the intensity of an optical signal is discussed in N. Imoto et al's article entitled "A non-linear optical fiber interferometer for nondemolition measurement of photon number", Optics Communications Vol 61 No2, Jan. 15, 1987 which employes a ring-type interferometer. In this arrangement a cw signal from a YAG 1.3 .mu.m laser is amplitude modulated to produce a data optical signal which data is to be tapped without reducing its intensity.
The data signal is transmitted through a first wavelength dependent mirror coupled into a silica optical fibre from which it exits to pass through a second wavelength dependent mirror.
A probe signal at 1.52 .mu.m impinges on a 50:50 beam splitter positioned so equal portions are directed to the first and second mirrors. These mirrors are reflective at 1.52 .mu.m and angled to direct the portions into opposite ends of the optical fibre. The two portions propagate round the ring defined by the beamsplitter and the two mirrors in opposite directions. They recombine at the beamsplitter a portion being transmitted and reflected interferingly to an optical detector adjacent the beamsplitter.
The probe signal portion which propagates co-directionally with the data optical signal has its phase modulated relative to the other probe signal by the data optical signal due to the optical Kerr effect. This phase modulation is measured by the non-linear optical detector as the varying interference between the two probe portions. The transmitted data is therefore detected without tapping any of the data signal.
The prior art QND tapping arrangements just described would be difficult to employ in a practical communications systems. There are several optical components which must be kept in accurate alignment if the data signal is not to be unnecessarily attenuated. There are several interfaces in the apparatus which will introduce some losses to the data signal even though no energy is directed to the optical detector.